Rain sensor device for detecting the wetting and/or soiling of a windscreen surface

ABSTRACT

A device for detecting the wetting and/or soiling of a windscreen surface, in particular in a vehicle, comprises a camera with a sensor having a plurality of light-sensitive pixels arranged as an array and adapted to be illuminated according to an illumination cycle, and having a focussing optic for a camera focus set to almost infinite. The device is further provided with a light source for illuminating a detection portion of the screen surface detectable by the camera. The light source may be switched on and off according to a predeterminable ON/OFF cycle. The ON/OFF cycle is synchronized to the illumination cycle of the camera sensor, and the wetting/soiling of the screen surface may be detected by comparing the image information from the sensor of the camera when the light source is turned on and when the light source is turned off.

TECHNICAL FIELD

The invention relates to an optical sensor for detectingsoiling/humidity on a pane, in particular a windscreen of a motorvehicle.

BACKGROUND

For controlling the windscreen wipers in dependence on the wettingcondition of a motor vehicle, windscreen sensors of variousconfigurations are known.

In some cases detection of the wetting condition requires that thesensor gets in direct contact with the humidity, e.g. in the case ofresistive or thermal sensors. Sensors which do not get in direct contactwith humidity are in most cases mounted directly at the inner windscreensurface and utilize the change in the refraction index at the wettedwindscreen surface (FIG. 1).

A light beam 6, in particular in the infrared range, is coupled by asensor element 4 at a certain angle into the vehicle windscreen 1 suchthat the light beam 6 is reflected as completely as possible at thesurface and can be received by a photo diode 4. When the surface iswetted by water drops, the refraction conditions at the surface aredisturbed such that the light portion measured at the photo diode 4varies as a function of the wetting condition.

The variation of the amplitude of the received light is evaluated forthe purpose of controlling the windscreen wipers.

Such as system which offers the advantage of complete insensitivitytowards foreign light is known from WO-A-95/01561.

In contrast to resistive, thermal or capacitive systems, optical rainsensors of the type described above were successful in the market. Acertain restriction with regard to the mounting location is thenecessity of direct optical contact with the windscreen inner surfaceand/or special measures to be taken for coupling the light beam in andout.

The employment of a video camera for detecting a wetting or soilingcondition on the windscreen outer surface is known from DE-C-198 03 644,DE-A-197 49 331, DE-A-197 04 818, U.S. Pat. No. 6,097,024 and U.S. Pat.No. 4,867,561. However, these cameras of these systems exclusively servefor detecting rain drops.

SUMMARY OF THE INVENTION

It is an object of the invention to allow detection of the wettingcondition of a pane surface from a distance with the aid of a camerawhose focus lies, as seen from the camera, far behind the pane surface.In particular, the invention is intended to allow employment of a camerafor detecting fogging/condensation and/or rain on a windscreen, saidcamera being provided for covering the area in front of the vehicle asseen in the driving direction.

According to the invention, for achieving this object a device fordetecting the wetting and/or soiling of a windscreen surface (on theinside and the outside), in particular of a vehicle, is proposed whereinthe device comprises:

-   -   a camera comprising a sensor having a plurality of        light-sensitive pixels arranged in the form of an array and        being adapted to be illuminated in accordance with an        illumination cycle, and a focussing optics for a focus of the        camera set to almost infinite, and    -   a light source for illuminating a coverage range of the        windscreen surface adapted to be covered by the camera,    -   wherein    -   the light source is adapted to be switched on and off in a        predeterminable ON/OFF cycle,    -   the ON/OFF cycle is synchronized with the illumination cycle of        the sensor of the camera, and    -   wetting/soiling of the windscreen surface is detectable by        comparing the image information from the sensor of the camera        with the light source being switched on and of.

Various embodiments of the invention are stated in the subclaims.

According to the invention, a video camera is used for detecting foggingand/or wetting of a windscreen of a vehicle.

Video cameras in vehicles are generally known and are used, for example,for covering an area in front of the vehicle. Thus, the vehicles can beautomatically controlled. The invention provides for a video camera, incase such a video camera is mounted in a motor vehicle, to beadditionally used for detecting fogging and/or wetting and soiling ofthe windscreen. Such a camera covers an area of e.g. 10 m to 50 m infront of the vehicle. In other words, its focus is adjusted toquasi-infinity. The camera “looks” through a relatively small area ofthe windscreen wiped by the windscreen wipers at the road as well as thearea at both sides of and above the road. Without any additionalmeasures taken, a rain drop is recognized by the camera only in anunsharp manner at this focal adjustment (quasi-infinite), which is ofadvantage in that otherwise the image information important for the(semiautomatic) vehicle control would be falsified or not detected.

According to the invention, wetting/soiling of the windscreen is madevisible to the camera by clocked illumination by means of light, whereinthe light source clock for this exposure purpose is synchronized withthe exposure frequency of the camera. Exposure of the camera iseffected, in dependence on the type, either pixel-, line- or image-wise.If the light source is e.g. switched on at half the exposure frequency,every second pixel, every second line and/or every second imagecomprises, besides the image information (brightness value) additionalbrightness information when in the coverage range of the pixel, the lineand/or the image concerned wetting of the windscreen is detected. Bycomparing the brightness values of adjacent pixels, lines or images ofsuccessive images or, more generally, by comparing the brightness valuesof pixels, lines and/or images including pure image information, withpixels, lines and/or images comprising, due to additional illuminationby the light source, also wetting information (additional brightnessvalues), it can be detected whether a wetting condition exists or doesnot exist. It is assumed that the image information does notsubstantially change despite their detection by the camera pixelssucceeding each other in terms of time. This assumption applies when theexposure frequency (image rating frequency) of the camera is comparablyhigh. This is already the case e.g. as from 50 images per second.

In an preferred aspect of the invention, light in the non-visiblewavelength range is used for the clocked additional exposure of thecamera (always exposure to the ambient light). This measure isadvantageous in that the passengers are not troubled by the clockedlight, and the generally used digital cameras are generally sensitive inthis wavelength range. Conventionally, the non-visible wavelength rangeis filtered out by such cameras since the cameras is to only “see”,during their application e.g. for covering the road, what is alsovisible to the human eye. The same situation prevails when a digitalcamera is used e.g. as a photo camera.

Appropriately, the additional clocked light source is sinusoidallycontrolled. This is of advantage with regard to electromagneticcompatibility.

In a further preferred aspect of the invention it is provided to detectwetting of the windscreen surface by filtering out and evaluating theON/OFF cycle frequency, by means of which the light source is adapted tobe activated, with the aid of a synchronous demodulator and/or a filterassembly.

Preferably, an image brightness signal can be obtained form the outputsignal of the sensor of the camera, namely by filtering the ON/OFF cyclefrequency out of this image brightness signal.

In the case of clocking of the light source in accordance with the pixelinterrogation frequency of the sensor of the camera, it is of advantagewhen the ON/OFF cycle of the light source is phase-shifted from line toline by 180°. Within the two-dimensional pixel field of the camera everysecond pixel would be exposed to the light of the additional lightsource, wherein the additionally exposed pixels are arranged from lineto line with a gap between them.

In the case of a light source clocked in accordance with the lineinterrogation frequency and/or the image interrogation frequency, theaforementioned phase shift is generally not necessary.

Preferably, it is further provided that for obtaining a signalindicating a wetting condition from the signal of the sensor of thecamera, the signals of the pixels of two successive lines of the sensorare subtracted. In this connection, it is further appropriate when thewetting signal is subjected to a filtering and/or synchronousdemodulation process.

For obtaining an image signal with superimposed wetting information (d.c. light portion), preferably the signals of two successive lines of thesensor are added.

For obtaining an image signal without wetting information, the lightsource is clock-synchronously changed over between two lines of thesensor of the camera lying one behind the other.

Preferably, it is further provided that within a time segment of theoutput signal of the sensor of the camera an averaging withoutswitched-on light source is effected, that for another time segment ofthe output signal of the sensor of the camera an averaging withswitched-on light source is effected, and that for obtaining a wettingsignal and/or a wetting value, the two average values are compared witheach other. This procedure is applied in particular when the evaluationas to whether a wetting/fogging/soiling condition exists is carried outon the basis of a complete camera image. If the average value of acamera image, in the case of exposure exclusively to the ambient light,differs from the average value of a following camera image, in the caseof additional exposure to the light source, by less than apredeterminable threshold value, no soiling and/or fogging/wettingcondition exists. If the difference in the average values remains thesame over a plurality of image sequences, a consistent wetting/bedewingcondition exists. This indicates that the windscreen is soiled e.g. byan insect impinging upon the windscreen. Finally, if the difference inthe average values varies over a plurality of image sequences, avariable soiling of the windscreen exists, which indicates rain orfogging.

In the procedure described above, a mutual comparison between theaverage value of the time segments without switched-on light source andthe average value of the time segments with switched-on light source isperformed by filtering the differential value and/or synchronousdemodulation for the purpose of detecting a wetting condition.

With regard to coupling the additional light into the windscreen it isof advantage when this light is directly coupled into the windscreen,wherein, when a wetting condition exists, a reflection towards thecamera is effected at the side opposite the coupling-in side. Thecoupling-in of the light is appropriately effected from the side intothe windscreen or via a prism into the surface of the windscreen.

Preferably, the electromagnetic radiation of the light source impingesupon that surface of the windscreen and is reflected there, on which awetting condition is detectable.

The concept according to the invention can be used for detecting of bothoutside wetting (rain) of the windscreen and internal fogging of thewindscreen. To be able to differ between the two conditions, it isadvantageous when a second light source is used whose light impinges bytotal reflection on the inside of the windscreen for the purpose ofdetecting any fogging. These two light sources are adapted to beseparately activated for the purpose of separate detection of a wettingcondition on the outside of the windscreen and a fogging condition onthe inside of the windscreen. Alternatively, the two light sources canbe simultaneously activated at different clock frequencies.

For detecting an outside wetting condition, the brightness valueobtained by exposure to the second light source and reflection at theinside of the windscreen towards the camera (possible in the case ofbedewing), is subtracted form the brightness value obtained byilluminating the windscreen with the first light source, which causesreflections at the inside and the outside of the windscreen, in order toobtain information on the outer wetting condition (rain).

Advantageously, it is further provided that the light from the firstlight source impinges by total reflection upon the inside of thewindscreen for detecting an internal fogging condition, and the lightfrom the second light source impinges by total reflection upon theoutside of the windscreen for detecting an outside wetting condition.

Finally, it is noted that it may be of advantage when the focussingoptics of the camera is set between almost infinite and the windscreensurface.

Alternatively, it is further possible to employ a detector for detectingcontrast steps in the signal of the sensor for sensing a wetting/soilingcondition on the windscreen surface.

BRIEF DESCRIPTION OF THE DRAWING

The invention and preferred embodiments thereof will now be described ingreater detail with reference to the drawings in which:

FIG. 1 shows a schematic representation of the setup of a conventionalrain sensor and its arrangement relative to a windscreen;

FIG. 2 shows a schematic representation of a rain sensor according tothe invention for contactless detection of inside and outside wettingand soiling of a windscreen;

FIG. 3 shows a comparison of the sensitivities of the human eye with thewavelength of electromagnetic radiation;

FIG. 4 shows a schematic representation for explaining the coupling-inof pulsed light into the camera when the windscreen is wetted;

FIG. 5 shows a time diagram for the brightness values of exemplarysuccessive pixels of the camera and the switching on/off of the lightsource;

FIG. 6 shows a first embodiment of a circuit of the rain sensoraccording to the invention;

FIG. 7 shows a graphic representation of the normal backgroundinformation of a pixel in comparison to the additional informationsupplied by clocked illumination;

FIG. 8 shows a second embodiment of the circuit of the rain sensor;

FIG. 9 shows an example of the signal pattern of successive pixels of acamera in the case of illumination exclusively by ambient light;

FIG. 10 shows the signal shown in FIG. 8 with additional clockedillumination for detecting a wetting condition;

FIG. 11 shows a signal pattern similar to that shown in FIG. 10, howeverfor an adjacent line and with the clocking of the additional lightsource offset by 180°;

FIG. 12 shows the differential signal shown in FIGS. 10 and 11 forillustrating a wetting condition of the windscreen;

FIG. 13 shows another embodiment of a circuit of the rain sensoraccording to the invention;

FIG. 14 shows an embodiment of the process for coupling clocked lightvia a prism into the windscreen;

FIG. 15 shows an alternative process of coupling light of the clockedlight source directly from the side via the edge of the windscreen intothe windscreen;

FIG. 16 shows an example of an arrangement of another light source fordetecting an internal fogging condition;

FIG. 17 shows a schematic representation of the functionality ofilluminating the inside of the windscreen for detecting internalfogging; and

FIG. 18 a last embodiment for illustrating the possible arrangement oftwo light sources at both sides of a windscreen for the purpose ofseparate detection of and outside and an inside wetting condition.

DETAILED DESCRIPTION

According to FIG. 2 a camera 7 (e.g. monochromatic CCD or CMOS camera)is arranged inside the vehicle such that it covers a portion of thewindscreen 1 in the area swept by the windscreen wiper.

The actual function of the camera 7 is to monitor the road at a largerdistance to the vehicle for the purpose of road, distance or tunneldetection. The camera 7 is normally arranged such that it looks frominside the vehicle through the windscreen 1 in forward direction,wherein the line of vision of the camera preferably is concordant withthat of the driver.

By corresponding evaluation of the detected brightness signals of theimage taken e.g. the position of the vehicle centrally to the median andthe shoulder of a road can be calculated and/or regulated.

For this purpose it is necessary to place the focal point, i.e. theportion of the optimum image definition, outside the vehicle windscreenin order to take up the image elements important for calculationpurposes, which are rich in contrast and show a high edge definition.This may e.g. be the area 10-50 m in front of the vehicle. This requiresthe camera 7 to be focused to almost infinite such that the windscreensurface and/or the wetting of the windscreen surface seem out of focusin the image. The function of the arrangement described here is todetect drops near the camera objective with simultaneous focusing of theimage definition to almost infinite.

Changes occurring directly on the windscreen surface, e.g. by wetting,are however, represented in a less sharply defined manner. This offersthe advantage that wetting of the surface does not affect, up to acertain extent, the representation at a distance of e.g. 30 m. This“unsharp” image of the rain drops is distributed to larger image regionssuch that the road continues to be represented nearly in a high-contrastmanner.

In this case, on the basis of the image brightness signal supplied bythe cameral small drops on the windscreen surface would be nearlyinvisible. A reduction of the aperture and thus an extended depth offield are not recommended since this would result in rain drops on thewindscreen surface and objects at a very large distance being renderedwith the same sharp definition.

“Disturbing information” produced by drops then superimpose the usefulinformation concerning the road and may lead to an incorrect estimationduring the computer-aided evaluation of the current driving situation.

If the image information of objects located at a large distance is notto be affected by wetting of the windscreen surface, if possible, it isadvantageous to select a wide objective aperture when focusing theobjective to almost infinite.

The depth of field is thus reduced, and when the camera is focused toalmost infinite, image details of the immediate vicinity are no longerdetected. Thus impinging rain drops have, up to a certain extent, hardlyany influence on the image quality of objects located at a largedistance.

Without taking any further measures, this arrangement hardly allowsdetection, in particular of small rain drops, from the camera-suppliedimage information with the aid of corresponding algorithms.

To be able to properly detect even smallest drops despite the “unsharp”image information, the arrangement described here utilizes the effectthat a video camera on silicon basis covers a much larger spectral rangethan the human eye. Thus, in contrast to the human eye, a camera isstill sensitive in the near infrared range, i.e. at wavelengths of700-1000 nm and/or even reaches its maximum sensitivity in this range.FIG. 3 shows the difference of the sensitivity range of the eye relativeto the sensitivity range of silicon.

Generally, during camera operation the infrared range is suppressed bybarrier filters. Infrared portions would otherwise occur in the videoinformation and alienate the brightness representation, to which the eyeis used, of known image information during representation on a monitor.

In the embodiment described here, an IR barrier filter is explicitlyomitted. This does however not matter since the image information is notrepresented on a monitor.

For detecting rain drops 2 on the windscreen surface 1 infrared light 23is radiated via e.g. an IR light source 22 at a certain clock ratio tothe pixel-, line- or image rating frequency into the window portionutilized by the camera 7.

This may be effected e.g. by infrared LEDs with a wavelength of e.g. 880nm, said LEDs radiating pulsed light approximately parallel to the lineof vision of the camera into the windscreen. To obtain betterefficiency, the windscreen should be not an IR barrier in this portion.

The IR light 23 is at least partly reflected by the rain drops on thewindscreen 1 back to the camera by multiple reflection (see light beam24 in FIG. 2), as schematically shown in FIG. 4.

Preferably, the IR light is not only reflected by the drops directly onthe windscreen, but also by drops in the air up to a certain distance tothe windscreen surface.

FIG. 5 shows a line-sequential scanning of the camera image over acertain number of pixels, e.g. 1 to 10.

For better understanding, a uniformly illuminated area was taken as abackground image (see signal pattern 25 in FIG. 5), said areacorresponding to the electric value of the image brightness of thescanned area.

At the time 26 the IR light source was connected, which results in thesignal pattern 27.

The additional value Δ IR now corresponds to the value of the IRradiation reflected by the drops on the windscreen. The value of the IRlight quantity Δ IR reflected by the drops thus statistically representsthe magnitude of the wetting of the windscreen surface.

Without wetting, this value ΔIR approaches 0 and considers only theconstant value of a possible reflection at the windscreen itself an/orthe soiling particles on the windscreen.

Since in the practice the camera of course supplies a constantlychanging image and thus changing line information, this method does notallow for reliable distinction between image information and valuechanges due to additional reflection by the rain drops.

The emitted IR signal (pulsing of the IR light source 22) is thus pulsedat a frequency which is, if possible, directly related to the pixel-,line- or image frequency (in the embodiment shown in FIG. 6 at half thepixel frequency). In this connection, the IR light source 22 is switchedon at every second pixel.

Synchronization of the IR emitting phase with the pixel-, line- or imagefrequency is not absolutely necessary but considerably facilitatesfurther signal evaluation.

In the embodiment (FIG. 6) the pixel clock 31 is reduced by divider 32to half the pixel frequency and supplied to the driver stage 28 for theIR source 22. Other ratios between exposure frequency of the camera 7and the clock rate of the IR light source 22 (e.g. 1:3, 1:4) are alsopossible.

This ensures that at every second pixel the IR source 22 is switched on.

If e.g. the interrogation time for one pixel is 50 ns, this results in asquare signal at the IR source having a frequency of 10 MHzcorresponding to 50 ns switch on time to 50 ns switch off time.

Of course, the pixel-relatedly scanned image and/or brightnessinformation in a line will not take the flat course shown in FIG. 5. Independence on the image content any value may occur at any time (e.g. asshown in FIG. 9).

In order to still separate the wetting information from the luminositysignal, the embodiment of FIG. 6 provides that the luminosity signal 9supplied by the camera 7 is transmitted through a band pass or high passfilter 29 having a pass range that is analogue with the frequency of theIR light source.

This band pass or high pass filter 29 separates the low-frequencycomponents of the image information. At the output of the filter 29, thesignal information primarily are the signal portions of the IR lightsource 22 reflected by the rain drops, present as AC voltageinformation.

A synchronous demodulator 33 continues to suppress randomly occurringspectral portions in the image signal, e.g. by scanning a periodiclight-dark surface with a frequency similar to that of the IR lightsource 22.

The wetting information 34 is present at the output of the synchronousdemodulator 33 as a DC voltage signal.

The frequency of the IR source 22 also appears as a spectral line in theimage signal information supplied by the camera 7 and can be excludedusing suitable means such as a simple suppression filter 30.

FIG. 7 illustrates the statistic spectrum of the scanned imageluminosity signal.

The spectral line 37 of the IR light source 22 is located at the upperend of the spectrum, i.e. in a region where the least energy portion ofthe image luminosity spectrum 36 is to be expected.

In the embodiment of FIG. 6, the “blurring” of the rain droprepresentation bears a positive effect in contrast with a “sharp” image.Whereas a sharply imaged and thus small point of reflection would forexample comprise only one pixel and would therefore be filtered out byfilter 29 like a signal step in the image information, a “blurred” pointof reflection comprises a plurality of pixels and thus create a clearsignal after transient oscillation of the filter 29.

With this arrangement, a rather accurate detection of drops is possible,but the separation of the high-frequency IR signal information from theimage luminosity information might possibly not yield the requiredaccuracy.

FIG. 8 illustrates a circuit diagram for a nearly perfect separation ofwetting and image luminosity signal.

Observing the image information line by line, it becomes clear that thesignal path generally shows negligible changes from one line to theother.

Clocking the IR light source 22 within even line numbers, e.g. in evenpixel numbers, and within uneven line numbers in uneven pixel numbers, asimple addition of the luminosity signal of one line and the invertedinformation of the previous line yields as a result a signal for thewetting of the screen that is nearly independent from the imageinformation. This is illustrated in FIGS. 9 to 12. FIG. 9 illustrates arandom image luminosity signal 9 over a period of 22 pixels. In theillustration of FIG. 10, the IR light source 22 was turned on in oneline upon illumination of the even pixels 2, 4, 6, . . . , and an imagesignal 43 with a modulation corresponding to the wetting is obtained.

One line later, the IR light source 22 was turned on upon illuminationof the uneven pixels, i.e. the pixels 1, 3, 5, . . . , and an imagesignal 44 with a modulation shifted by 180° with respect to the firstline is obtained (FIG. 11).

The difference between both image signals 43 and 44 (s. FIG. 12) nowyields the pure wetting information 45. The same represent a “rain drop”in the area of the 9 and 10, which, however, seems to be distributedover a lot more pixels due to the blurredness of the image on thesensitive camera surface.

In the embodiment of FIG. 8, the luminosity signal 9 of the camera 7 issupplied to a delay circuit 121 with the delay time of one horizontalline. The differential signal 34 obtained from the luminosity signaldelayed by one line and from the non-delayed luminosity signalcorresponds to the wetting of the screen surface. Upon wetting, analternate signal with half the pixel clock frequency is present at theoutput of the differentiating stage 13.

For an additional increase of the detection security, the synchronousdetector 33 used in the embodiment of FIG. 6 may be added.

In the embodiment of FIG. 8, using a second circuit arrangement, thepure luminosity signal without the wetting information can be obtained.The switch 38 switches synchronously to the pixel clock rate between thedirect luminosity signal 89 and the luminosity signal delayed by oneline such that always only pixels are detected for which the IR lightsource has been turned off.

Thus, the pure luminosity signal 39 without the wetting information ispresent at the output of the switch 38.

Wetting Detection with Cameras Having a High Frame Rate

Of course, other switching sequences of the IR light source 22 can beused to detect the wetting of the screen surface. For example, the IRlight source 22 may be turned on during the entire time of scanning apicture, while the IR light source 22 is turned off for the next image.

With a fast frame rate, the image signal amplitude or its valueintegrated over the scanning time will change only negligibly from oneimage to the next so that a significant signal pattern results uponwetting.

FIG. 13 illustrates an embodiment of an arrangement for animage-sequential detection of the wetting of a screen surface 1.

The camera is supplied with the pixel and frame rate by the clockgenerator 31. Without wetting, the output signal 9 of the camera 7 willbe almost the same from one image to the next, while, with wetting, asignificant IR source synchronous signal pattern will again appear.

In the integrator 40, the total average value of the luminosity signalof a respective image is obtained. At the end of each image, the valuememory 41 receives the total average value of the luminosity amplitudethereof.

Thus, when the screen surface is wetted, a frame-rate synchronous ACvoltage is present at the output of the value memory 41.

The filter 42 is tuned to the frame rate frequency and, upon wetting,supplies a corresponding output signal from its output, which, after asynchronous demodulation in the synchronous demodulator 33 and acorresponding evaluation, may be used to control the windscreen wiper.

To obtain an image signal free of wetting signal components, of courseevery second image can be used for which the IR light source 22 had notbeen turned off.

The overall luminosity of a camera image of a camera with a high framerate changes insignificantly from one image to the next, or uniformly,e.g. when driving into a tunnel. Here, no output signal is generated atthe output of the filter 42. Only when the screen surface is wetted andan image-synchronous change in the amplitude is caused thereby, will acorresponding output signal be generated.

In principle, any optional switching sequence of the IR light source 22can be used, as long as a differentiation between periods illuminated bythe IR light source 22 and periods not illuminated by the IR lightsource is possible.

Coupling the IR Source From the Camera

Coupling the light source is most simply effected by light irradiatedapproximately in the line of sight of the camera 7.

Since a part of that light can be scattered in all direction by awetting, a part of the reflected light could possibly also hit thedriver or other passengers.

Therefore, the irradiated light should have a wavelength that is notvisible to the human eye. This is a range from 700 nm, e.g. IR at 880 nm(see FIG. 3).

Given a mechanical shielding of the diffusely reflected light radiationfrom rain drops on the windscreen surface towards the driver/passenger,of course, any wavelength can be used.

Another method of coupling is obtained by a direct coupling of the lightinto the screen 1, e.g. when using a prism 46 as illustrated in thearrangement of FIG. 14. Here, the light only has to be coupled orinjected into the screen, but not out from the same. Coupling the lightout to the camera 7 is then effected by the diffraction at thedrop-shaped wetting (as schematically illustrated in FIG. 4).

FIG. 15 illustrates a lateral coupling of the light source 22 into thevehicle screen. Due to total reflection within the screen 1, the lightstays entirely in the screen as long as no wetting allows for atransition of the light into a drop and thus causes a change of theangle of total reflection. In this case, a part of the light isdeflected in all directions, i.e. also to the camera.

The wetting detection presented up to now mainly reacts to externalsurface wetting, it also being possible to detect internal wetting, i.e.by condensing breath. This is less true for an irradiation in parallelto the line of sight of the camera, but clearly effective for a lateralcoupling or injection according to FIG. 14 or FIG. 15.

Detection of Internal Deposit/Fogging

It is often desired to be able to detect internal deposit/foggingindependent from rain drops on the outer surface of the screen.

Through a corresponding arrangement of two light sources, it may bedetermined whether the wetting is an external wetting, e.g. rain drops,or an internal fogging, e.g. condensing breath. Based upon thisdetermination, the blower or the windscreen wiper, for example, may becontrolled separately.

To detect the interior fogging, a second light source 47 is arrangedsuch that its light directed to the inner surface of the screen does notenter the screen when the screen is not wetted (dewy), but is possiblytotally reflected (see FIG. 16).

When external wetting occurs, the light beam of the second light source47 is not influenced. Only upon internal wetting 49 is the light beam ofthe light source 47 scattered diffusely so that part of the light alsoreaches the camera 7.

The light source 47 can preferably be driven with an own clock frequencyor alternately with the IR light source 22 to differentiate between aninternal fogging and an external wetting.

It should be observed that an internal fogging also has effects on thedetection of an external wetting, if both light sources are located inthe interior of the vehicle.

Subtracting the value generated by the detection in the interior fromthe value of the external rain drop detection, which also includes thevalue of the interior detection, the actual value for the exteriorwetting of the screen is obtained.

If it is possible to arrange a light source outside the vehicle screen(e.g. in the frame of the screens or a screen shoulder of the vehiclebody, the value of the inner fogging and the value of the externalwetting can be obtained entirely separated from each other (see FIG.18).

List of reference numerals 1 front screen 2 wetting, rain drops 3housing 4 transmitting element 5 receiving element 6 light path 7 camera8 objective 9 image signal, 9d delayed image signal 10 line of sight ofcamera 11 visual angle of camera 12 lag element 13 subtraction stage 14differentiated image signal 15 window comparator 16 digital wettingsignal 17 image signal “road” 18 differentiation stage 19 drop detection20 background signal 21 threshold values 22 light source, LED 23 emittedlight beam 24 light beam reflected by drop 25 image signal without lightsource turned on 26 difference with light source turned on 27 imagesignal with light source turned on 28 driver stage 29 band pass filter30 suppression filter 31 clock generator 32 divider 33 synchronousdemodulator 34 wetting information 35 filtered image information 36spectrum of image signal 37 spectrum of wetting signal

1. Device for detecting the wetting and/or soiling of a windscreen surface, in particular in a vehicle, comprising a camera with a sensor having a plurality of light-sensitive pixels arranged as an array and adapted to be illuminated according to an illumination cycle, and having a focusing optic for a camera focus set to almost infinite, and a light source for illuminating a detection portion of the screen surface detectable by the camera, wherein the light source is switched on and off according to a predeterminable ON/OFF cycle, the ON/OFF cycle is synchronized to the illumination cycle of the camera sensor, and the wetting/soiling of the screen surface is detected by comparing the image information from the sensor of the camera when the light source is turned on and when the light source is turned off.
 2. Device of claim 1, wherein one pixel, one pixel line or all pixels of the camera sensor are illuminated per illumination cycle and wherein the ON/OFF cycle is synchronized with a pixel-scanning frequency, a line-scanning frequency or an image-scanning frequency of the camera sensor.
 3. Device of claim 1, wherein the light source emits electromagnetic radiation in a wavelength range no visible to the human eye.
 4. Device of claim 1, wherein the light source is sinusoidally controllable.
 5. Device of claim 1, wherein a wetting of the screen surface is detectable by filtering out and evaluating the ON/OFF cycle frequency with which the light source is driven, using a synchronous demodulator and/or a filter arrangement.
 6. Device of claim 1, wherein an image luminosity signal is obtained from the output signal of the camera sensor by filtering the ON/OFF cycle frequency from this image luminosity signal.
 7. Device of claim 1, wherein the ON/OFF cycle is phase-shifted by 180° from one line to the next.
 8. Device of claim 1, wherein, for obtaining a signal indicating wetting from the signal of the camera sensor, the signals of the pixels of two successive lines of the sensor are subtracted.
 9. Device of claim 8, wherein the wetting signal is subjected to filtering and/or synchronous demodulation.
 10. Device of claim 1, wherein, for obtaining an image signal with superimposed wetting information, the signals of two successive lines of the sensor are added.
 11. Device of claim 1, wherein, for obtaining an image signal without wetting information, the light source is switched synchronously to the switching clock between two successive lines of the camera sensor.
 12. Device of claim 1, wherein, within a, period of the output signal of the camera sensor, an, averaging without activation of the light source is effected, wherein, for another period of the output signal of the camera sensor, an averaging with activation of the light source is effected, and wherein both average values are compared to obtain a wetting signal or a wetting value.
 13. Device of claim 12, characterized by a mutual comparison between the average value for the periods without light source activation and the average value for the periods with light source activation by means of filtering the differential value and/or synchronous demodulation for the purpose of detecting the wetting.
 14. Device of claim 1, wherein the light from the light source is coupled directly into the screen and, given a wetting, a reflection towards the camera occurs on the side opposite the coupling.
 15. Device of claim 14, wherein the coupling of the light is effected laterally into the screen or with a prism into the surface of the screen.
 16. Device of claim 1, wherein the electromagnetic radiation of the light source impinges with total reflection on that surface of the screen on which a wetting is to be detected.
 17. Device of claim 1, wherein the light source is a first light source the device further comprising a second light source whose light impinges under total reflection on the inside of the screen to detect wetting thereon.
 18. Device of claim 17, wherein both light sources are driven separately for a separate detection of wetting on the outside of the screen and fogging on the inside of the screen.
 19. Device of claim 17, wherein both light sources are driven simultaneously using different clock frequencies.
 20. Device of claim 17, characterized by the subtraction of a value of the inner reflection by the second light source from the value of the inner and outer reflection by the first light source in order to obtain a value for the external wetting.
 21. Device of claim 17, wherein the light of the first light source impinges under total reflection on the inner side of the screen for the detection of interior fogging and the light from the second light source impinges under total reflection on the outer side of the screen for the detection of an external wetting.
 22. Device of claim 1, wherein the focusing optics of the camera are adjusted between almost infinite and the screen surface.
 23. Device of claim 1, characterized by a detector for the detection of contrast steps in the signal from the sensor for detecting the wetting/soiling on the screen surface. 